Electric vehicles typically use one or more traction motors for propulsion. In some such vehicles, electrical energy is stored in on-board energy storage devices, such as batteries and ultra-capacitors, and fed to an inverter where direct current (DC) power is converted to alternating current (AC) power. The AC is then fed to multi-phase (typically 3-phase) AC motors that drive the wheels of the vehicle. Electric propulsion is becoming increasingly common in material transport vehicles, including underground mining vehicles, replacing diesel powered vehicles in the interest of complying with environmental and safety regulations, towering emissions, and improving mine air conditions.
Electric vehicles, however, require periodic charging of the on-board energy storage devices. While some electric vehicles utilize removable battery packs that are swapped out when they reach a low state of charge, other electric vehicles have energy storage devices that are permanently affixed to the vehicle, and which require a connection to a supply of electrical power for recharging. In either case, in a mine or other industrial facility, it may be impractical to move the vehicle and/or energy storage device outside the facility for charging. Therefore, the facility may be outfitted with charging stations, which are connected to the power distribution system of the facility for receiving a supply of electrical power for charging electric vehicle energy storage devices.
Operations in mine and other industrial facilities present unique challenges from a power supply standpoint. Mines, for example, are typically outfitted with a local power distribution system that is connected to the public utility grid and/or local power generating equipment. The local power distribution system provides power for lighting, ventilation equipment, mining equipment and machinery, and the like. For example, the power distribution system may be used to supply power to dragline excavators, power shovels (e.g., electric mining shovels), continuous miners, and other machinery or electrical devices, which are known to present large electrical loads.
Indeed, with many of the loads on the distribution system being large motor loads, there are often large inrush currents during startup. In addition to large load swings, these loads can create large step changes when they are turned off or on. When the local power distribution system is weak, such as when the is connected to the main grid by long transmission lines or is part of a microgrid, the ability of the local system to manage these large load steps within the required time is significantly lesser, leading to grid instability. Even in strongly connected grids, such large load swings are unfavorable for normal grid operation and could lead to nuisance tripping of protective devices and excessive wear and tear on electrical equipment.
Moreover, in connection with power distribution to mining machinery, reactive power is needed within the mine at the site of such mining equipment and machinery. The frequent intermittent operation of the machinery's large electric motors, however, may cause voltage drops across the long feeder cables between the electrical substations (that form a part of the local distribution system) and the machinery. Accordingly, maintaining adequate voltage is one of the more challenging problems in the mining industry, and is oftentimes the primary constraint on mine expansion from the point of power delivery to the mining operation. This is due to the fact that both motor productivity and life decrease with decreasing voltage. In certain cases, maintaining minimum acceptable voltage sets determines the maximum allowable cable length and, as a result, the maximum physical extent of the mine.